Wireless communications devices need to detect and classify electromagnetic waveforms in the vicinity of the device. As one example of this need, a particular wireless device may need to determine if other devices are outputting electromagnetic radiation that could interfere with transmissions of the particular wireless device. The interferer could be another communication device such as a cordless phone, or 802.11 device. However, the interferer might not be a communication device. For example, a microwave oven could cause interference.
As another example of this need to detect and classify electromagnetic waveforms, a wireless device may need to detect radar signals to comply with government regulations. Throughout the world, various communication regulatory bodies allow wireless communications devices, such as 802.11 devices, to operate in certain frequency bands subordinate to other transmitters. For example, in the United States an “unlicensed” wireless device is allowed to operate in the 5.250-5.350 and 5.470-5.725 GHz bands, provided that the unlicensed device employs dynamic frequency selection (DFS) to vacate the band if a radar signal is present in the band. Prior to and during transmission in these bands, the unlicensed wireless device must monitor for radar waveforms and either vacate the band or flag the band as unavailable for present use by the unlicensed wireless device.
In order to detect and classify electromagnetic waveforms such as interfering signals and radars, the wireless device can first extract features of the electromagnetic waveforms. These features could be time-domain features, such as those that pertain to signal pulses. One example of a time-domain feature is a given pulse's time difference of arrival (TDOA), which is the difference in arrival time between the given pulse and the previous pulse. A parameter that is related to the TDOA is the pulse repetition interval (PRI), which is defined as the time between successive pulses. The PRI is typically calculated based on the TDOA of a train of pulses. The PRI may be constant over time or time-varying. For example, the PRI of a microwave oven may become shorter over time. Another time-domain feature is the pulse width. In addition to these time domain features, frequency domain features such as carrier frequency could also be extracted from the electromagnetic waveform. Based on the extracted features, the electromagnetic waveform can be classified or identified by comparing the features with expected characteristics.
However, if there are errors in feature extraction, then the electromagnetic waveform could be mis-classified. For example, fluctuations in the magnitude of the electromagnetic waveform, as received, may cause pulses to be missed. Also, background noise could lead to “detection” of a false pulse. Both of these errors could lead to a substantial error in calculation of the PRI. Furthermore, timing jitter due to noise can lead to a substantial difference between the TDOA at the receiving device and the time difference between pulses at the transmitting device.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, the approaches described in this section may not be prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.